The present invention relates to removing olefins and dienes from aromatic streams. In particular, the present invention relates to a method for selectively converting undesirable components such as dienes and olefins to provide a substantially purified aromatic product.
Aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis) and can be used as feedstocks in a variety of petrochemical processes, such as para-xylene production from an aromatic stream containing benzene, toluene and xylene (BTX), or toluene disproportionation. However, aromatic streams often contain hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds, such as anthracenes, which can cause undesirable side reactions in these processes. Therefore, these hydrocarbon contaminants must be removed from reformate-derived aromatic streams before they can be used in other processes.
Improved processes for aromatics production, such as that described in the Handbook of Petroleum Processing, McGraw-Hill, New York 1997, pp. 4.3-4.26, provide increased aromatics yield but also increase the amount of contaminants. For example, the shift from high-pressure semi-regenerative reformers to low-pressure moving bed reformers results in a substantial increase in bromine reactive contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). The number of grams of bromine absorbed by 100 grams of a hydrocarbon or a hydrocarbon mixture indicates the percentage of double bonds present. Thus, when the type and molecular weight is known, the contents of the olefin can be calculated. The Bromine Indices (i.e., numbers) of the hydrocarbon feeds and products are measured to determine the change in composition. Molecular sieves and clay treating have been used to reduce the Bromine Indices of various hydrocarbon products.
The clay treatment of hydrocarbons is widely practiced in the petroleum and petrochemical industries. Clay treating is used to remove impurities from hydrocarbons in a wide variety of processes. Most often, the heavier hydrocarbons, that is those having six or more carbon atoms per molecule, are subjected to clay treating rather than lighter hydrocarbons. One of the most common reasons for clay treating these materials is to remove olefinic materials, sometimes called xe2x80x9cbromine contaminants,xe2x80x9d in order to meet various quality specifications. As used herein the term xe2x80x9colefinic compoundxe2x80x9d or xe2x80x9colefinic materialxe2x80x9d is intended to refer to both mono and diolefins. Olefinic materials may be objectionable in aromatic hydrocarbons at even very low concentrations of less than a few parts per million. For example, in the manufacture of nitration grade aromatics including benzene, toluene and xylenes, it is essential to remove these olefinic materials from the feedstock.
Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams, such as benzene, toluene and xylene (xe2x80x9cBTXxe2x80x9d) streams, by contacting the aromatic stream with acid-treated clay. Other materials, such as zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material and, consequently, relatively inexpensive. However, zeolites used for this purpose are usually synthesized and are, therefore, more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities in the feedstream, since BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, it is still a significant expense and it is not uncommon for large aromatic plants to spend close to a million dollars a year on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants from aromatic streams is impractical unless their cycle length can be increased.
The high cost of catalysts and the loss of production when the process is shutdown to replace the spent catalyst has created a need for an efficient and cost effective method for removing contaminants from reformate-derived aromatic streams. The present invention solves this problem by advantageously using a combination of catalytic reactors and clay treaters to more efficiently remove contaminants from reformate-derived aromatic streams while extending the life of the catalysts.
In accordance with the present invention, a method is provided for the treatment of aromatics reformate to remove olefins therefrom by contacting the reformate with a molecular sieve to convert the olefins to alkylaromatics. Preferably, the molecular sieve is a zeolite, most preferably a large pore size zeolite. The reformate can be contacted with a hydrotreating catalyst prior to contacting with the molecular sieve to substantially convert dienes contained therein to oligomers and to partially convert the olefins to alkylaromatics. In addition, the reformate can also be clay treated after contacting with the molecular sieve to substantially convert the remaining olefins to alkylaromatics.
In another embodiment of the present invention, a method is provided for the treatment of aromatics reformate to remove dienes and olefins. The method includes: contacting an aromatics reformate containing dienes and olefins with a hydrotreating catalyst to substantially convert the dienes to oligomers and to partially convert the olefins to alkylaromatics; contacting the reformate with a molecular sieve to further convert the olefins to alkylaromatics to provide an olefin depleted product, wherein less than 30 percent of the olefins in the aromatics reformate remain in the depleted product; and clay treating the olefin depleted product to substantially convert the remaining olefins to alkylaromatics. In a preferred embodiment, more than 95 percent of the dienes and the olefins in the aromatics reformate are converted. Using the Bromine Index as a measure of olefin content, the present invention reduces the Bromine Index of an aromatics stream from about 300 to 1,000 to below 100.
The hydrotreating catalyst has a metal component selected from the group consisting of: nickel, cobalt, chromium, vanadium, molybdenum, tungsten, nickel-molybdenum, cobalt-nickel-molybdenum, nickel-tungsten, cobalt-molybdenum and nickel-tungsten-titanium. The support for the catalyst is conventionally a porous solid, usually alumina, or silica-alumina but other porous solids such as magnesia, titania or silica, either alone or mixed with alumina or silica-alumina may also be used, as convenient. A preferred hydrotreating catalyst is a nickel molybdenum/alumina.
The olefin removal is preferably carried out using a large pore size zeolite as a molecular sieve, wherein the zeolite is ZSM-4, ZSM-12, mordenite, ZSM-18, ZSM-20, zeolite beta, Faujasite X, Faujasite Y, USY, REY and other forms of X and Y, MCM-22, MCM-36, MCM-49, MCM-56, M41S or MCM-41. The preferred zeolites are MCM-22 and zeolite beta, most preferably a self-bound MCM-22 zeolite.
After the aromatics reformate has been hydrotreated and contacted with a molecular sieve to remove the dienes and at least 70% of the olefins, it is clay treated to substantially remove the remaining olefins. The clay treating is carried out at a temperature of from about 100 to about 240xc2x0 C. and at a pressure of from about 100 to about 300 psig. Any clay suitable for processing hydrocarbons can be used, preferably Engelhard F-24 clay, Filtrol 24, Filtrol 25, and Filtrol 62, Attapulgus clay or Tonsil clay, with Engelhard F-24 clay being the most preferred. In one embodiment of the present invention, the aromatics reformate is clay treated after the hydrotreater and before the molecular sieve reactor.
In a preferred embodiment, the method of the present invention also includes separating the oligomers from the reformate after contacting with the hydrotreating catalyst and prior to contacting with the molecular sieve. This allows the alkylation of olefins in the molecular sieve reactor to be carried out more efficiently. However, it is within the scope of the present invention for the oligomers to be separated downstream of the molecular sieve reactor and the clay treater.
It has been found that the best mode for practicing the present invention employs a nickel molybdenum/alumina hydrotreating catalyst, a self-bound MCM-22 zeolite and Engelhard F-24 clay. This combination of catalysts and clay efficiently removes the contaminants from the aromatics reformate and extends the life of the catalysts.
By using both a zeolite bed and a clay treater, the present invention takes advantage of the high conversion rate of zeolites and the low cost of clay to reduce catalyst consumption, extend catalyst life and reduce the system operating costs.